Transmission coupling circuit



R. W; ,MASTERS TRANSMISSION COUPLING CIRCUIT Aug. 146.,

Filed June 9, 1945 Z PTH Patented Aug. 6, 1946 TRANSMISSION COUPLING CIRCUIT Robert W. Masters, Erlton, N. J., assignor to Raclio Corporation of America, a corporation of Delaware Application June 9, 1943, Serial No". 490,379

This invention relates generally to high frequency transmission networks and more particularly to a transmission network for relatively isolating a plurality of generators of dierent frequencies which are coupled to a common load circuit.

' Networks of the type described have wide utility in radio transmitter antenna circuits wherein a plurality of transmitters having different output frequencies are coupled through a common transmission line to either common or adjacently disposed antennas.

Briefiy, the instant invention comprises means whereby two transmitters having the same or different output frequencies are coupled to a balanced transmission line in a manner which provides satisfactory transmission of both frequencies to a load circuit connected to the transmission line, while eifectively isolating the transmitters from each other. If the feeder line from the higher frequency transmitter is not already balanced With respect to ground, it may be coupled to the balanced transmission line through any conventional coupling circuit such as, for example, a resonant re-entrant network of the type illustrated herein. One terminal of the second transmitter is coupled to both conductors of the balanced line, while the remaining terminal is grounded.

The balanced transmission line includes separate capacitors in series with each of the conductors thereof. A center tapped tuned circuit such as, for example, two parallel disposed quarter wave conductors, is connected across the balanced line intermediate the capacitors and the higher frequency input coupling circuit. The remote ends of the quarter wave lines are grounded and a shorting bar between the quarter wave lines may be employed for adjusting the circuit to be non-receptive to the higher transmitter frequency. A shielded two wire transmission line is connected between the output terminals of the balanced transmission line and a load device such as, for example, an antenna.

A second pair of parallel disposed quarter wave conductors are connected together at the ends remote from the balanced line and also connected across the balanced line intermediate the series capacitors and the output terminals. A second shorting bar may be provided for tuning the second pair of quarter wave conductors to antiresonance at the higher output frequency. The combination of balanced line, series capacitors, and tuned quarter wave lines comprises a 'nnetwork which insures satisfactory transmission of 2 Claims. (Cl. 178-114) the higher frequency signals through the input coupling circuit and balanced transmission line to the output circuit and the load. One terminal of a lower frequency transmitter is connected to the common terminal of the output quarter wave tuned lines. The remaining terminal of the lower frequency transmitter is grounded. The shielding means on the output two conductor line is also grounded.

It will be seen that both terminals of the low frequency transmitter are substantially at ground potentialwith respect to the higher frequency signals, and coupling of the lower frequency signal source to the higher frequency transmitter is slubstantially limited by the balanced connection to the balanced transmission line and the relatively high capacitive reactances intermediate thereof.

'Thebalanoed shielde'dV line comprises the two conductors for the higher frequency output connection to the load, while both of the output line conductors taken together and the grounded shield comprises the output connection from the lower frequency transmitter to, for example, a second load.

Essentially, the shunt connected tuned lines may be considered as reactances for providing suitable impedance matches between the balanced transmission line, the load circuit and the separate transmitter feeder lines. In order to minimize refiections in the feeder line from the higher frequency transmitter, the input impedance of the balanced line should be substantially resistive and equal to the surge impedance Z'O of the higher frequency feeder line. The first tuned reactor X2., the second tuned reactor Xe and the two series capacitors Xb should therefore be adjusted to bear the following relations:

where the output impedance Zz of the balanced line is substantially resistive and equal to the load resistance Rz. i

It will therefore be seen that by proper choice of the sizes and signs of the reactances Xe, Xb and Xe, any resistive output impedance Zz maybe transformed efficiently With any predetermined input resistance R1.

If, however, the output impedance Zz is not purely resistive, but includes some reactive component, the shunt reactance element Xc of the circuit may be calculated to a value, in excess of the value provided by Formula 3, which would precisely parallel-resonate the reactance of the load impedance Zz, thus providing an effective load of pure resistance.

Since the load impedance Zz varies as a function of frequency, it is essential that both reactances Xz, and Xe be adjustable in order to satisfT the requirements of Equations 21 and 3. The reactanes of the series capacitors Xb are made large enough so that, even though fixed, equation (1) is always satisfied. It should be understood that the variable reactors X8, Xe may;be inthe form of tuned parallel conductors as illustrated herein, or in the case of lower frequency transmission circuits, in the form of lumpedltuned reactors of conventional types.

It should be understood that either the higher or the lower frequency transmitter may be connected to the feeder line which is balanced with respect to ground and also that the system will be operable with transmitters of substantially'the same output frequencies. Likewise, the system is 'applicable to transmission of any frequencies whatever, if suitable circuit componentsareselected 'for the particular Operating frequency band.

Among the objects of the invention are to:provide an improved methodof and means forccou.` pling a plurality of generators having .different output frequencies to a common Vtransmission line in v'a manner which efiectively isolates the generators from each other. Another 'object of the invention isto provide an improved method of andmeans for connecting two generators having different output lfrequencics to a -balanced transmission network connected to'a common transmission line whereby satsfactory transmission of both vfrequencies may be accomplished while effectively isolating the generators'from each other. A further object of the invention "is to provide an improved circuit for Vcouplinga plu rality' of generators having different output frequencies through a ar network to `a common output transmission line. An additional object of 'the invention is to provide an improved circuit for coupling two generators having different output frequencies to a Vbalanced transmission `ocircuit connected to a common output transmission line' wherein circuits offering high impedance Vto the currents of the vhigher frequency c permit Fsatisfactory coupling of the lower frequency transmitter to the balanced transmission line.

The invention will be described in further detail by Vreference to' the accompanying vdrawing ;of which the single figure'thereof is a vschematic circuit diagram of a preferred embodiment.

Referring to the drawing, -a high frequency radio transmitter l having an output concentric line 2 is connected through a resonant re-entrant network 3 of the 'type commonly known as a "bazooka or line .coupling transformer." A transmission line 4, balanced with respect to ground, is connected to the output terminals of the resonant re-entrant network 3. Series capacitors 5,.`6 are inserted in each of the conductors of the balanced transmission line The output terminals 1, -8 ofV the balanced transmission line vll are connected to the conductors of a shieldedtwo wiretransmission line-9. A pair of conductors IG, H, having, for example, lengths of the order of slightly in excess of any odd multiple of 1A wave length at the frequency of the high frequency transmitter, are connectedV together and grounded at one end thereof. The remaining ends of the quarter wave conductors ll), are connected to different conductors of the balanced transmission line 4 intermediate the capacitors 5, 6 and the re-entrant network 3. A shorting bar l2 is connected between the quarter wave conductors Ill, ll for tuning the circuit to offer high impedance to thecurrents of the output frequency of the high frequency transmitter I.

A second pair of parallel disposed conductors L3, VhLhaving, for example, lengths of the order of any cdd multiple of 1A wave length at the operating frequency of the high frequency transmitterl, are connected together, and are also connected to thedifferent conductors of the balanced `transn'iission line 4 at points intermediate the capacitors 5, 6 and the output terminals 1, 8, respectively. A second shorting bar 15 is connected between the second. parallel disposed conductors E3, l'for tuning the circuit to offer high impedance to the currents of the frequency of the high frequency transmitter. The common terminal of the second pair of tuned parallel conductors |3, M is :connected'to one output vterminal of a lower frequency transmitter 15. The remaining output terminal of the lower frequency transmitter IB is grounded.

Theshield l'l of the two Wire shielded output line -9 is grounded. The two conductors of the two wire ouputline 9 are connected to a first load device or antenna H. A neutral point IS on the first load device 1B is connected to one terminal of a second load or antenna '20. The remaining terminal of the second load or antenna 20 is connected to the shicld ll of the two Wire shielded line 9. The particular type of load device or antenna system employed does not comprise any par-t of applicant's invention, and it should'be understood that any known means may beem- 'ployed for connecting the two conductors and Shield of the two fwire shielded transmission line Sato one or more load devices.

It will be seen `that the resonant re-entrant network 3, the balanced transmission line 4 and the two conductors of the shielded transmission line 9 provide an efiicient coupling'circuit between the higher frequency transmitter l and the first load device l8l Since the two pairs of tuned parallel disposed lines IB, H and IS, IQ, respectively, offer high impedance to the currents of the higher frequency transmitter I, the connection of the lower frequency transmitter IG to the common connection of the second pair of tuned lines is eifectively isolated from the higher frequency transmission circuit. Similarly, the ungrounded connection of the low frequency transmitter' I 6 to the common connection of the second pair of tuned lines 13, 64 provides a dual path through the conductors |3', M and the two conductors of the shielded output line 9 to the second load device 28. The low frequency transmitter is thereforeeifectively isolated fromthe high frequency transmitter since the low frequency connection is substantially at ground potential with respect to the high frequency transmitter, and the series capacitors 5, G offer a high impedance path to the lower frequency energy.

Thus the invention described comprises an improved method of and means for coupling two generators having different output frequencies to a balanced 1r network for transmitting energy from both generators to a common output line while effectively isolating the generators from eachother.

I claim as my invention:

1. A circuit for relatively isolating two generators having different output frequencies, said generators being coupled to a common transmission line including balanced means for connecting said line to a source of high frequency oscillations, second balanced means for connecting said line to a source of relatively lower frequency oscillations, means for making said high frequency connections at points on said line at balanced potential with respect to said lower frequency generator, means including capacitors serially connected in said line for making said lower frequency connections at second points on said line substantially at ground potential for said higher frequency generator and means for connecting said line to a load device.

2. A circuit for relatively isolating two generators of different frequencies, said generators being coupled to a common transmission line, including a balanced transmission line, means for connecting said higher frequency generator to said line, a first balanced tuned'circuit having a grounded neutral point connected across said line, a second balanced tuned circuit connected across said line, capacitors serially inserted in the conductors of said line intermediate said first and said second tuned circuit connecti/onsV thereto, means for connecting' one terminal of said lower frequency generator to the neutral point in said second balanced tuned circuit, means for coupling said balanced line to a load circuit balanced with respect to ground, and means grounding the remaining terminal of said lower frequency'generator.

ROBERT W. MASTERS. 

